FIG. 2 shows a conventional power supply regulating apparatus for a heater. The power supply regulating apparatus 20 for a heater has an electricity receiving terminal table 2 connected to an AC power source 1 at the input terminals of the apparatus, and has a terminal table for distribution 6 connected to a heater 7 at the output terminals of the power supply regulating apparatus. A power source breaker 3, a power source transformer 4, and a power control thyristor 5 as a power regulator are connected between the electricity receiving terminal table 2 and the terminal table for distribution 6. A thermometric thermocouple 8 is disposed in the heater 7.
Power is received at the electricity receiving terminal table 2 from the AC power source 1, and the power is supplied to the power source transformer 4 via the power source breaker 3. The power transformed by the power source transformer 4 is controlled by the power control thyristor 5 and supplied from the terminal table for distribution 6 to the heater 7. The heater 7 is heated thereby, and the temperature of the heater 7 changes. The heater temperature is measured with the aid of the thermocouple 8 and inputted to a temperature controller 9. The temperature controller 9 calculates the difference between the set temperature and the measured temperature measured with the aid of the thermocouple 8, and calculates the amount of electric power to be supplied to the heater 7 in accordance with the temperature difference. The result of the calculation is converted to a phase control amount and outputted as a control signal from the temperature controller 9 to the power control thyristor 5. The power control thyristor 5 supplies power to the heater 7 in correlation with the timing of the control signal.
In this manner, the power supply regulating apparatus 20 for a heater senses the heater temperature, subsequently determines the timing for outputting control signals with the aid of the temperature controller 9, controls the phase of the power control thyristor 5 in accordance with the timing, and thereby brings the temperature of the heater 7 to the set temperature.
The method of controlling the phase is shown in FIG. 3. FIG. 3A shows a power waveform of an AC power source, and FIG. 3B shows the power-controlling thyristor control signal for controlling the power control thyristor. In each cycle of the AC power source in the phase control method, the power control interval A is the interval that begins with the generation of a power-controlling thyristor control signal and ends when the power waveform is at zero volts, and the reactive power interval B is the interval that begins at zero volts and ends with the generation of a control signal. There is a need for a power source in which the maximum power is higher than the power required when the temperature is stable. Therefore, the effective power when the temperature is stable is about 60 to 80% of the maximum power, and the rest is reactive power. Hence, the efficiency of such a power source is poor.
Attempts have been made to improve this situation, including the adoption of zero crossing control in which the reactive power is substantially not generated, and increasing the ratio of the effective power to 85% or higher with the aid of a phase advance capacitor for improving the power factor.
Zero crossing control has the same circuit implementation as in FIG. 2. Rather than adopting a thyristor as a power control element, however, an SSR (solid-state relay) is generally adopted. This approach is different in that the content of the control signal is changed. The method of this zero crossing control is shown in FIG. 4. FIG. 4A shows the power waveform of the AC power source; and FIG. 4B shows the power-controlling SSR control signal for controlling the SSR. An ignition method is adopted for turning on the SSR when the power waveform is zero volts. In this method, the prescribed time (A+B) of the AC power source is a single cycle (single cycle time), the power control interval A is the interval in which a power-regulating SSR control signal is outputted and [the SSR] (*1) is energized, and the non-energized interval B is any other interval in which power is not consumed. Zero crossing control acts only to turn the power source on or off, and reactive power is substantially not generated.
FIG. 5 shows the control method using a phase advance capacitor. The solid line in FIG. 5a shows a supply-side AC power source waveform W1, and the dotted line shows a control-side power source waveform. FIG. 5b shows a control signal of the power control thyristor. The reactive power interval B is long when the supply-side AC power source waveform W1 indicated by the solid line is controlled by the control signal. Therefore, the power control in the case of the phase angle P1 does not rise above 70%, for example. However, the reactive power interval B is reduced by an amount commensurate with the advance of the phase angle P2, the apparent power factor is improved, and power control increases to 90%, when the control-side power source waveform W2, which is indicated by the dotted line and whose phase has been advanced by the phase advance capacitor, is controlled with the aid of the control signal of the power control thyristor.
In the case of zero crossing control, however, an SSR having a relatively large on-voltage in comparison with a semiconductor for high speed power control such as IGBT (Insulated Gate Bipolar Transistor) is used as the power control element, and the heater temperature response is degraded. In the case of a phase advance capacitor, power regulation for limiting the profile is required until maximum power is reached because of the compensation of the phase advance capacitor. Because the phase is advanced in this case, an open phase is produced when a maximum power is abruptly applied. The system is therefore made difficult to use.